A throttling device creates a large pressure drop by forcing flow through a restriction such as a valve, porous plug or capillary tube.

For steady flow through an adiabatic throttle with no shaft work and negligible changes in kinetic and potential energy, the energy balance gives

$$h_1=h_2.$$

Throttling is therefore approximately isenthalpic, not isentropic. The pressure drop is irreversible and entropy generally increases.

The temperature can rise, fall or remain nearly unchanged depending on the fluid and inlet state. For an ideal gas, enthalpy depends only on temperature, so constant enthalpy implies approximately constant temperature. Real fluids can behave differently.

A throttle extracts no useful shaft work from the pressure drop. That distinguishes it from an expansion turbine, which attempts to convert part of a pressure and enthalpy decrease into useful work.

Throttling illustrates an important second-law lesson: a pressure drop can destroy work potential while conserving total energy.